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Nanoantennas Used to Study Nonlinear Optical Effects

Photonics.comJun 2011
STUTTGART, Germany, June 6, 2011 — For the first time, researchers are using nanoantennas to focus laser light on oscillating nanoparticles to investigate ultrafast nonlinear optical effects.

When a short laser pulse is focused on nanoparticles, they heat up very briefly and start to vibrate. But even the best microscopes cannot resolve these nanoparticles, which are therefore very difficult to study. Now Markus Lippitz, assistant professor at the Max Planck Institute for Solid State Research, and his PhD student Thorsten Schumacher have achieved a breakthrough using a gold nanoantenna that has already been applied successfully as a nanosensor by Harald Giessen, a professor at the 4th Physics Institute of the University of Stuttgart. (See: Nanoantenna Enhances Plasmonic Sensing)

By analyzing the oscillations, scientists can learn a lot about the nanogold: Does it vibrate like a piece of steel — i.e., the volume is important — or does it rather vibrate like a balloon — i.e., the behavior is dominated by the surface. This new research field is called nanomechanics, and Lippitz is considered one of its leaders. In his laboratory, there is a complicated experiment of lasers, mirrors and nanoparticles. This optical setup resembling a strobe light allows the researchers to watch the oscillations. If a nanoparticle is in the path of a laser beam, its intensity is modulated slightly. Lippitz measures these fine oscillations with a special procedure that he developed.

Assistant professor Markus Lippitz (left) with his PhD student Thorsten Schumacher. (Images: University of Stuttgart)
Lippitz' dream is to investigate the mechanical properties of the smallest nanoparticles. “The surface-to-volume ratio would then be huge, and we would expect new nanomechanical properties,” he said. To get one step closer to this dream, he placed a small antenna near the tiny particle. This nanoantenna focuses the laser light very tightly on the nanoparticle under examination. Consequently, the light modulation due to the nanomechanical vibrations is very efficiently coupled back into the laser beam.

“This is the first time that someone uses nanoantennas to investigate ultrafast nonlinear optical effects. The whole thing works like a mobile phone in which the antenna makes that the electromagnetic waves are effectively coupled into the small electronic circuits of the phone,” Lippitz said.

The triangular nanoantennas focus the laser light on the small gold dot in the center, which starts to oscillate and to modulate the transmitted laser beam.
Lippitz sees a huge potential for his new method: “In the future, we will be able to put the smallest nano-objects of a few nanometers in diameter in the focal point of a nanoantenna and study them using nonlinear optical processes of only a few femtoseconds' duration. Then we can make movies on the nanoscale, using the most extreme slow motion. Not only can we investigate nano-objects such as semiconductor quantum dots, but also chemical and biological objects, such as molecules and viruses,” he said.

Giessen added: “The work by Markus Lippitz is a result of the very successful collaboration between the Max Planck Institute and the University of Stuttgart. Markus Lippitz is an excellent young scientist who also supports the Stuttgart Excellence Initiative in the Graduate School, ‘Advanced Condensed Matter Science.’ ”

The work of Lippitz was funded by the state of Baden-Württemberg and by the Deutsche Forschungsgemeinschaft and the Federal Ministry of Education and Research.